Thanks for reaching out Mark J1,
It sounds like your Apple TV is going to sleep while you're using AirPlay to stream music to it from your Mac. We can help.
Since you've already tried updating the tvOS software, let's start by attempting to Restart the Apple TV. Many times restarting a device can fix intermittent issues.
If that still doesn't work, you may want to contact Apple to report the behavior, but you can check out these steps as a workaround in the meantime:
You can set how long Apple TV waits before automatically going to sleep.
1. Open Settings
on Apple TV.
2. Go to General > Sleep After and choose an option.
You can connect with Apple directly using either Get Support or if you're in a region without direct support options, you can also Contact Apple for support and service.
Hope that helps.
Take care.